dinsdag 21 september 2010

The Green Mountain [A terra-formed forest]

The story of Ascension Island's transformation from a totally treeless "cinder" as Charles darwin found it in July 1836 to the rich vegetation of its rain-forming cloud forest at Green Mountain begins with an English botanist named Joseph Hooker who visited the island in 1843.

Listen to D.M. Wilkinson (The parable of Green Mountain: Ascension Island, ecosystem construction and ecological fitting):

...at the request of the British Admiralty, made recommendations to ‘improve’ the Ascension environment. Hooker presented four main suggestions (Duffey, 1964):

1. Planting trees on the mountain which he considered ‘of the first importance as thereby the fall of rain will be directly increased’.

2. Developing the formation of deeper soils by encouraging more vegetation to grow on the steeper slopes.

3. Planting the more promising areas in the lower valleys with drought adapted trees and shrubs.

4. Introducing suitable crops into gardens on Green Mountain.

The thinking behind this scheme is strikingly similar to much more recent ideas for creating life-friendly conditions on Mars (so-called terraforming), where the idea ‘should not just be about creating a new environment for life through force majeure, but about finding ways to allow life to create a new environment for itself’ (Morton, 2002, p. 298). The idea that trees promote rainfall, and so improve their own environment, dates from measurements of transpiration rates at the start of the eighteenth century and became an influential idea amongst many administrators of the British Empire (Grove & Rackham, 2001). However, on Green Mountain, the trees probably mainly increase occult precipitation by trapping moisture from the regular mists.

For several years after Hooker’s visit, consignments of plants were sent to Ascension every month and, after 1850, twice a year, from England each November and from the Cape of Good Hope each May (Duffey, 1964). Such was the success of this scheme that Alistair Hardy could describe the vegetation of Green Mountain in the 1920s as ‘good and hearty’, writing that ‘tall eucalyptus trees now lined the road, flowering shrubs, conifers and palms of many kinds appeared, and sheep grazed on the slopes of grass in between patches of almost dense jungle’ (Hardy, 1967, p. 124).
Today, much of the higher parts of Green Mountain are best described as cloud forest, contrasting strikingly with Darwin’s complaint of a landscape ‘entirely devoid of trees’. Indeed the mountain is dominated by introduced plant species. Hooker later had second thoughts about the conservation implications of his ‘terraforming’ scheme, writing: ‘The consequences to the native vegetation of the Peak will, I fear, be fatal, and especially to the rich carpet of ferns that clothed the top of the mountain when I visited it’
& The lesson is:
The Green Mountain system is a spectacular example of ecological fitting. It shows that coevolution is not necessary to the development of a complex ecosystem, although it should be noted that this does not necessarily mean that it is never important. It should also make us more sceptical of arguments based on long histories of coevolution that are sometimes used to explain high levels of tropical biodiversity. On Green Mountain, where humans have solved the plant-dispersal problems, the system has gone from species poor ferndominated hillsides to species-rich cloud forest in around 150 years.

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